INDIAN MARINE SURVEYS. 25 



the survey launch ahead to sound whenever there was a doubt as to 

 which channel the main stream had adopted for the dry season. 

 The possession of a gun and a bluejacket's crew were a very service- 

 able help to them when sounding ahead of the other vessels. Com- 

 mander Carpenter and his party then piloted the flotilla up to 

 Bhamo, having first taken the pilots over the shallowest portions and 

 examined the latest-formed channels. A complete sketch survey 

 was made of the river from Tbayetmyo to Bhamo, and was checked 

 by astronomical observations at 20 positions, forming altogether a 

 valuable addition to geography. The survey party returned to 

 Rangoon and rejoined the " Investigator " towards the end of 

 January, having received the thanks of General Sir H. Prendergast 

 for the skilful assistance which they had rendered to the Irawacli 

 war flotilla. 



The next survey taken up was that of the Mergui archipelago, 

 the Admiralty chart of which (by Captains E,oss and Lloyd of the 

 Indian Navy) was found to be very correct, the only defect being an 

 insufficient number of soundings. The islets are all steep, and many 

 are mere pinnacles, which makes it probable that similar dangerous 

 pointed rocks exist below water. The Great Western Torres islands, 

 immense heaps of boulders overgrown with foliage, were visited, 

 and their position tested by angles to known peaks and found to be 

 1-J- miles out. 



About this time a fresh datum for the low-water level on Indian 

 charts was resolved upon. Previously, -the soundings used to 

 indicate the depth at the average lowest tides of all the lunar 

 fortnights during the year, but as this had been proved by the 

 observations of Major Baird to give in some cases more water 

 than actually existed, owing to the considerable difference on the 

 west coast of India between night and day tides, and on the Bast 

 and Burmese coasts between winter and summer ocean level, it 

 was decided that all soundings and tide tables should be reduced to 

 the lowest low water of the year, provided it was not phenomenal, 

 e.g., brought about by an earthquake or cyclone. The only exceptions 

 to this rule was to be made at Karachi, Marmagao, and in the River 

 Hugh, where the harbour authorities sound their own ports and 

 prefer their own reduction datum. This, however, was a matter 

 which would not confuse the mariner, as at such ports pilots are 

 obligatory. 



